In an impromptu press conference at the White House, the president said his own campaign's television and radio advertisements against the presumptive Republican presidential nominee never crossed the line.

Pablo Martinez Monsivais / AP

President Barack Obama speaks in the White House briefing room in Washington Aug. 20.

"If you look at the overall trajectory of the campaign and the ads that I have approved and that are produced by my campaign, you'll see that we point out sharp differences between the candidates, but we don't go out of bounds," Obama said.

Some of the sharpest instances of negativity during the 2012 campaign -- Vice President Joe Biden's comment before a mixed-race audience that Republicans and Wall Street would "put y'all back in chains" -- prompted Romney to condemn Obama for running a "campaign of division and anger and hate."

Campaigning with running mate Paul Ryan on Monday in New Hampshire, Romney accused the Obama campaign of lying by saying his tax plan would result in higher taxes.

At an impromptu White House news conference, President Obama comments on GOP Mo., Senate candidate Todd Akin's remarks about rape, Mitt Romney's refusal to release more than two years' worth of tax returns, and the unrest in Syria. Watch the entire news conference.

"It seems that the first victim of an Obama campaign is the truth, and it has been sad and disappointing," Romney said.

The Obama campaign's assertion was based on a nonpartisan tax group's analysis that middle class families would effectively face a higher tax burden if Romney eliminated many popular deductions as part of his comprehensive tax reform.

The former Massachusetts governor's grievances don't end there, either. The Romney campaign was particularly incensed by an ad produced by Priorities USA, a pro-Obama super PAC, which suggests a man's wife died from cancer because he lost his insurance after was laid off from his job at a company owned by Bain Capital, the private equity firm founded by Romney.

"I don't think that Gov. Romney is somehow responsible for the death of the woman that was portrayed in that ad. But keep in mind: this is an ad that I didn't approve, I did not produce, and, as far as I can tell, has barely run -- I think it ran once," Obama said.

But the president also challenged Romney's own advertising on welfare, which assert that Obama had "gutted" the centerpiece of the 1995 welfare reform law, which tied benefits to seeking work.

"You've got Gov. Romney creating as a centerpiece of his campaign this notion that we're taking [the] work requirement out of welfare, which every single person here who's looked at it says is patently false," Obama said. "They can run the campaign that they want, but the truth of the matter is, you can't just make stuff up."

The president's comments Monday come as POLITICO published a new e-book describing internal tension in the Obama campaign regarding the trajectory of the re-election campaign, and how the sharper edges of Obama's 2012 effort square with the more optimistic banner of "hope and change" in 2008.

Republicans have sought to make an issue of disillusionment with Obama's shift; the Republican National Committee produced an ad last week accusing the president of running a campaign of "anger and division."

But the president said he was satisfied with the manner of his campaign.

"I feel very comfortable with the fact that, when you look at the campaign we're running, we are focused on the issues and differences that matter to middle class families all across America," Obama said. "And that's exactly the kind of debate the American people deserve."